The invention relates to an arrangement in an apparatus for analyzing, on the basis of radiation absorption properties, one or more gas component included in a gaseous medium from an origin, said apparatus comprising: a radiation source; a measuring chamber provided with radiation transmissive windows for receiving a gas mixture from at least the respiratory air of a person; a first detector, which receives the radiation passed through the gas mixture in said chamber; and between the radiation source and the detector a first optical interference filter, which has a first radiation transmission wavelength band and which is tiltable relative to the passing radiation beam and providing a wavelength shifting of the first radiation transmission band before reaching said first detector directed towards a radiation output surface of said first interference filter. The invention also relates to methods at a process for determining one or more gas component included in a gaseous medium from an origin, whereupon an arrangement defined is used. A specific application of the inventive arrangement and method is to analyze the respiratory air of a person and e.g. the alcohol content of the respiratory air.
Specifically the invention relates to Evidential Breath Analyzer (EBA) with a device, which measures alveolar ethanol concentration at exhaled air from a person. EBA""s are used normally for forensic purposes e.g. detection of intoxicated drivers. The idea to use breath instead of blood has been well known for decades but only recently has international standardization work done in this field. International Organization of Legal Metrology (OIML) has defined error limits for EBA measurements in its recommendation OIML R 126 xe2x80x9cEvidential breath analyzersxe2x80x9d, Edition 1998. The recommendation states the of level physical or physiological disturbances, which are allowed, and not allowed respectively, to affect the final result. One of such influencing factor is ambient air (e.g. the atmosphere of a room or a car) surrounding the person whose exhaled air is to be measured. Ethanol or other alcohols as well as some other substances in gaseous state can contaminate the ambient air and cause erroneous result.
One widely used technology for EBA""s is infrared (=IR) spectrometry. Most IR spectrometers use well-known Lambert-Beer formula [C=kxc2x7ln(I0/I)] to detect the concentration C of a known substance. In the formula k is a constant, the value of which depends on the device, the substance to be analyzed and the units. The ratio of the intensities I0 and I can be determined by various methods. One possibility is to construct and use a reference channel, which is always without any interfering substance, and another possibility is to arrange multiple IR detectors each of them with an IR filter having a transmission band different from those of the other IR filters in front of other IR detectors.
Modern apparatuses, utilizing radiation absorption properties on the basis of non-dispersive methods, for determining one or several gas components, are built without reference gas chamber or reference detectors mainly due the cost reasons. In such device determination of concentration is done in two phases. At the first phase the intensity of radiation is measured when a measurement chamber is filled with non-absorbing reference gas. This intensity is noted as I0 indicating the lack of absorbing substances. This measurement is followed by second measurement where radiation goes through the gas mixture to be inspected/analyzed. This intensity is noted as I. Concentration of inspected substance is calculated using the Lambert-Beer formula disclosed above. The possible weakness of this method is its dependency of reference gas purity-non-purity. If an ambient air is used as reference gas the presence of such substances which are absorbing in the wavelength band used will distort the concentration result. Room or especially car atmosphere may contain and sometimes contain harmful absorbing substances such as isopropanol or octane and the like.
It is known to eliminate the effect of said type of substances on the concentration measurement results of ethanol in case these harmful substances are present in the exhaled air of a person. Publication U.S. Pat. No. 4,587,427 discloses an apparatus and a method utilizing a rotating filter wheel with several side by side arranged filters having narrow transmission bands different from each other, which filters are brought one by one in the IR radiation beam to cause successive signals. The actual ethanol concentration is then determined from these signals with a specified calculation procedure. Exactly the same results as above, but without moving parts, are also achieved by using beam splitters, which are half mirrors, and separate filters in the beams portions divided by the beam splitters. This kind of an apparatus is disclosed in publication U.S. Pat. No. 5,693,945. The beam splitters does not alter the wavelength distribution of the radiation transmitted through and reflected from the half mirrors. A multitude of filters and possible half mirrors make these two alternatives very bulky, expensive and not applicable to mobile use. In a further publication U.S. Pat. No. 5,793,043 discloses an apparatus and a method utilizing a single optical interference filter with a specified transmission band in the radiation beam. This one filter is inclined to different angles in respect to the IR radiation beam, whereupon the wavelengths of the transmission band received by a single detector are shifted as compared to each other causing accordingly different successive signals from said one detector. The actual ethanol concentration is then determined from these different signals with a calculation procedure. These publications do describe means for eliminating the effects of harmful substances in the step of determining the ethanol=alcohol content in the sample of exhaled air, but do not provide any means to detect these harmful substances in a reference gas sample nor provide any means to eliminate the erroneous measurement results caused by these substances if present in the reference gas. These publication do not mention this reference as problem at all, and so the problem remains unsolved.
In a publication GB-2 313 198 another type of measurement device is disclosed comprising an electrochemical measurement cell for determining the ethanol content of the exhaled air and a separate measurement chamber with an IR detector as a reference measurement device as well as a reservoir for storing a calibration substance. Ethanol is used as said calibration substance. During operation the breath air of a person is first of all analyzed for alcohol content using the electrochemical measurement cell, secondly this electrochemical measurement cell is flushed with ambient air, thirdly this cell is allowed gradually fill with the ethanol from the reservoir until the same concentration value is reached as determined in the breath sample of the person, and further this mixture of the ambient air and the ethanol from the reservoir is analyzed in measurement chamber said using the IR detector thereof. This latter measurement result is utilized as a reference to the primary alcohol measurement result received by the electrochemical measurement. In this method the same problems as described above remain unsolved, because the calibration gas mixture used as a reference has ambient air as a solvent, and hence both the sample of the exhaled air and the sample of calibration gas mixture contain all of those harmful substances of the ambient air if present. Because a reservoir for calibrating substance is required and two separate measurement chambers/cells are also needed, this apparatus would necessarily have a very large size. According to this publication these harmful substances are detected and their effect eliminated neither in analyzing the breath sample nor in analyzing the sample of the calibration gas mixture. This publication neither describe the problem discussed nor disclose any means, by which it could be possible to find out these harmful substances or eliminate their effect, and so the method of this publication is much more inferior and provide much lower reliability than the methods of the two publications described above.
The object of the invention is to achieve a method and an arrangementxe2x80x94in an apparatus for analyzing, on the basis of radiation absorption properties, concentration of one or more gas component included in a gaseous medium from an originxe2x80x94with which it could be possible to get a reference gas without a need of storing it or any part of it in a reservoir. A second object of the invention is to achieve a method and an arrangement to detect the presence of substances, which may alter absorption properties of the gaseous medium to be analyzed within the spectral range utilized so as to render the measurement result to be erroneous and non-allowable. A third object of the invention is to achieve a method and an arrangement, which could be effectively used in portable or mobile apparatuses. A fourth object of the invention is to achieve a method and an arrangement, which provide conditions to draw up and produce a measuring device of aforesaid type, which has a high reliability and accuracy in spite of the possible presence of the above mentioned absorption altering substances in the ambient air of that origin, a gaseous medium from which is to be analyzed. A fifth object of the invention is to achieve a method and an arrangement, which could be put in practice using as few critical. optical components as possible. A further object of the invention is to achieve a method and an arrangement, which are applicable to devices for analysis of the breath of a person or an animal whether inhaled or exhaled, and which are also proper for evidential breath analyzers.
The above-described problems can be solved and the above-defined objects can be achieved by means of an arrangement of the invention which is characterized by what is set forth in the claims and by means of a method of the invention which is characterized by what is set forth in the claims.
According to the invention the problem described is surprisingly solved by measuring the radiation reflected from an interference filter in a device provided with this kind of a filter, which has a transmission wavelength band and is inclinable to different angles in respect to radiation beam, for identifying the type of and/or determining the concentration of at least one gas component of a gas mixture on the basis of radiation passed through said gas mixture and through said interference filter. According to the further features of the invention all essential harmful substances present in the air ambient to or surrounding the object or origin, from which the gas mixture to be analyzed originates, can be detected surprisingly by using just that ambient/surrounding air as a reference gas. According to still further features of the invention the same detector, which is used for measuring said reflected radiation, is surprisingly also applicable to detect an inclined position of said interference filter and hence utilized to calibrate the interdependency between the inclination angle and the wavelength band shift of said interference filter. Because the minimum amount of optical components and the versatile use thereof and because no special reference gas or an storage thereof is needed, the inventive arrangement and method can be used to have a very compact apparatus. Further advantages of the invention are reach with its other features.